To demonstrate repulsion and attraction of charges
1. Place a watch glass on a few drops of oil to reduce friction (Fig. 20.1).
2. Rub a Perspex rod with wool and balance it on the glass.
3. Take another Perspex rod rubbed with wool and bring it near the first rod.
4. The rod on the glass will repel and begin to rotate away from the other rod.
5. Rub a polythene rod with wool and note the opposite effects as the rod on the glass is now attracted to it. This demonstrates that like charges repel, but unlike charges attract.
watch glass on oil FIG. 20.1 NOTE
When rubbed with woollen cloth, polythene becomes negative as it gains electrons. Perspex becomes positive as it loses electrons. To remember which rod has which charge, think ‘polythenegative’.
Perspex rod
Conductors and insulators
It is too simple to say that electricity flows through a conductor and does not flow through an insulator.
In reality, there is no such thing as a perfect insulator. Electricity can move from one place to another if there is enough potential to do so. For example, think how far lightning travels from clouds to ground through the medium of air. Air would normally be considered a very good insulator.
A conductor is a medium that allows electric charge to flow through it easily.
An insulator is a medium that does not allow electric charge to flow through it easily.
FIG. 20.2 Lightning travels from clouds to ground through the medium of air, even though air is a good insulator
Charging objects by induction
When charging objects by induction, we cause materials to become charged by forcing the electrons to move a certain way.
We’ll look at two methods of charging by induction. One uses two insulated spheres, the other just one.
